A double-cylinder door lock is a type of security device primarily found on residential doors. Homeowners often use these locks in conjunction with other types of security products, such as lever locks or handlesets. They can also be used as the primary locking mechanism on patio or balcony doors.
A traditional single-cylinder deadbolt features a keyhole on the exterior side of the door and a thumbturn on the interior side. Homeowners can unlock these devices by either turning a key from outside, or by turning the thumbturn by hand from inside. Double-cylinder deadbolts have keyholes on both sides of the opening. They can be locked or unlocked only with a key, and no thumbturn is present.
Double-cylinder door locks are primarily found on doors that have glass panels nearby. Without these locks, burglars could easily break glass panels in or around the door and reach inside the home to unlock the door. When a double-cylinder lock is present, even if a burglar breaks the glass, he will be unable to unlock the deadbolt from inside without a key. Any door that has a lock within an arm's reach of glass panels, sidelights or windows can benefit from a double-cylinder deadbolt. There is no real benefit from using these locks on solid wood or metal doors.
Installers insert double-cylinder locks through small holes bored into the door. They are typically placed above the door handle, usually close to eye level. The interior and exterior halves of the lock are pressed tight to the face of the door, and usually connect through a series of built-in connecting rods. Screws or other fasteners are installed on the interior side of the door to keep the lock in place while protecting the screws from tampering. A bore hole must be cut in the door frame to accommodate the lock bolt. Many homeowners will find these locks are fairly easy to install without the help of a professional.
If you decide to purchase a double-cylinder lock, there are several types to choose from. They are categorized by different grades, with Grade 1 models among the most secure and durable. Choose a lock finish that matches the rest of your hardware to create a cohesive look. For the highest level of security, look for locks marked as "pick-resistant" or "high-security." These models contain special mechanisms and cylinder designs that make it nearly impossible to pick the lock. For ease of use, consider choosing a deadbolt with a motion-sensor activated light. This helps to guide your key into the lock at night, and often includes a separate light that alerts you once the deadbolt has been locked.
Double-cylinder locks pose a danger to the home's occupants during a fire or other emergency. While trying to escape through the door, occupants will be forced to take extra time to use a key to unlock the deadbolt. This can delay egress, particularly if the key can't be located. It is critical that the key is kept within close reach of a double-cylinder lock at all times for safety. Many life safety codes, including the widely used National Fire Protection Association (NFPA) Life Safety Code 101, prohibits the use of this type of lock on egress doors. Check your local building codes before installing this type of lock.